Field of the Invention
The present invention relates to a medical diagnostic. More particularly, the present invention relates to optical interrogation configurations for investigating tissue modification in real-time during medical procedures.
Description of Related Art
The presence of a lesion in a field of normal tissue can often be detected by the changes in the way light interacts with the different tissue components. For example, the visual assessment of a surgeon is dominated by the change in the scattering of the light in the visible part of the spectrum by the different tissue components. The light in the near infrared (NIR) part of the spectrum can also detect such differences arising from changes in the structure and biochemical composition of the tissue components. A well recognized property of the NIR light is that it can penetrate deeper into the tissue, on the order of a few cm, mainly due to reduced absorption by blood but also due to reduced scattering. The dependence of the mean penetration depth of the photons as a function of wavelength in different tissue components forms the basis of the U.S. patent application Ser. No. 11/414,009 (the parent case) entitled “Fiber Optic Evaluation of Tissue Modification,” incorporated herein by reference and describing the use of NIR spectroscopy for lesion assessment. Specifically, this application provides a novel approach to characterize critical parameters in real time, particularly suitable for application during radio frequency (RF) ablation of cardiac tissue, by incorporating the use of a fiber-optic probe on a typical ablation catheter. RF ablation is commonly used to treat atrial fibrillation, a heart condition that causes abnormal electrical signals, known as cardiac arrhythmias, to be generated in the endocardial tissue resulting in irregular beating of the heart. The RF energy is delivered locally via ablation electrode catheters that can be inserted percutaneously under local anesthesia into a femoral, brachial, subclavian, or internal jugular vein and positioned in the heart. Current methods have limited effectiveness in measuring lesion formation parameters in real-time or associated adverse conditions.
The parent case enables critical parameters of the process leading to the formation of the lesion to be evaluated in real time including such parameters as catheter-tissue proximity, lesion formation, depth of penetration of the lesion, cross-sectional area of the lesion in the tissue, formation of char during the ablation, recognition of char from non-charred tissue, formation of coagulum around the ablation site, differentiation of coagulated from non-coagulated blood, differentiation of ablated from healthy tissue, and recognition of microbubble formation in the tissue for prevention of steam pop. These assessments are accomplished by analyzing the spectral characteristics of the diffusely reflected light from the tip of the ablation catheter via the incorporation of fibers to deliver the illumination and collect the backscattered light.
The most frequent cause of cardiac arrhythmias is an abnormal routing of electrical signals generated in the endocardial tissue near the atrial or ventricular walls. Catheter ablation can be used to treat cases when arrhythmia cannot be controlled with medication, or in patients that cannot tolerate these medications. Using an ablation catheter or similar probe having an energy-emitting element, usually in the form of radiofrequency (RF) energy, a sufficient amount of energy is delivered in the location of suspected centers of this electrical misfiring, leading to the formation of a lesion. These lesions are intended to stop the irregular beating of the heart by creating non-conductive barriers between regions of abnormal electrical activity. Successful treatment depends on the location of the ablation within the heart as well as the spatial characteristics of the lesion.
Attaining contact of the catheter with the tissue is critical for the formation of the lesion. Various methods have been explored as means to provide confirmation of establishing a proper contact during surgery. These means include monitoring of the electrical impedance between the catheter electrode and the dispersive electrode (which utilizes the difference in resistivity between blood and endocardium) along with monitoring the temperature at the tip of the catheter. However, in current practice, these methods do not provide a reliable tool to determine proper contact of the catheter with the tissue. As a result, experience and skill of the electrophysiologist performing the procedure play a major part on the clinical outcome.
The effectiveness of lesion therapy is evaluated by a post ablation monitoring of the electrical signals produced in the heart. If it is determined that signals responsible for arrhythmia are still present (suggesting that the lesion was not adequately formed), additional lesions can be created to form a line of lesions to block passage of abnormal currents. However, there is currently no method to assess in real time how the lesion is forming. The ablation process can also cause undesirable side-effects such as charring of the tissue, localized blood coagulation, and vaporization of tissue water that can lead to steam pocket formation and subsequent implosion (steam pop) that can cause severe complications. All these side effects can be mitigated by adjusting the RF power of the catheter if the operator is aware of their development. Clearly, being limited to post ablation evaluation is undesirable since correction requires additional medical procedures while the surgeon has minimal knowledge regarding the development of undesirable ablation side effects. Thus, there is a need for the development of a guidance tool that could help evaluate the lesion formation parameters in real time as it is being formed in the tissue.
Thermal coagulation of myocardium leads to significant changes in its optical properties. For the case of myocardium coagulation via RF ablation, Swartling et al. reported that the changes in the optical properties in the near infrared (NIR) spectral region include an increase of the scattering coefficient (≈5% higher), a smaller decrease in the scattering anisotropy factor (≈2% lower) and an increase in the absorption coefficient (≈20% higher). We hypothesized that these changes in the optical properties of the RF ablated cardiac tissue can be used to provide in vivo monitoring of lesion formation parameters. Considering that absorption by blood and myocardium in the NIR spectral region is minimal, we postulated that in vivo monitoring may be based on NIR light scattering spectroscopy. Such a method could be employed through the vascular system, preferably as a fiber-optic attachment to the RF ablation catheter.
The parent case teaches a method for the evaluation of lesion formation via RF (or other type of) ablation in real-time using near infrared (NIR) light scattering spectroscopy. The ablation catheter was modified to incorporate spatially separated light emitting and receiving fibers that may be in contact with the tissue as the lesion is formed at the tip of the catheter. Spectral analysis of the light collected by the receiving fiber allows detection of key parameters such as, contact of the catheter with the tissue, onset of lesion formation, depth of penetration of the lesion and, formation of char or coagulum during the ablation.